1. Field of the Invention
The present invention relates to a method and apparatus for polygon mirror scanning, and more particularly to a method and apparatus for polygon mirror scanning capable of performing a high speed polygon mirror rotation without causing a change of balance.
2. Discussion of the Background
In general, a dynamic pressure air bearing motor using a radial dynamic pressure air bearing is adopted to various high speed rotating apparatuses including motors for rotating a polygon mirror used in a laser recording apparatus, a hard disc, an optical disc, etc. The radial dynamic pressure air bearing is a relatively small ring-shaped air space formed between a fixed shaft that forms a radial dynamic pressure bearing and a rotary body having an inner circumferential surface held for free rotation by an outer circumferential surface of the fixed shaft. When the rotary body is driven for rotation, the fixed shaft and the rotary body do not contact each other due to a dynamic air pressure produced in the ring-shaped air space.
To implement a high print speed and a high definition with an image forming apparatus using a laser recording mechanism such as a digital copying machine, a laser printer, etc., it is needed to drive a polygon mirror accommodated in the laser recording mechanism to rotate at an extraordinary high rotational speed, for example 40,000 rpm or higher, and at a high definition.
Conventionally, a ceramic-made dynamic pressure air bearing that allows no shaft locking production, such as galling and burning, is used as a bearing for a high speed polygon scanner. A ceramic is used as a material of a rotary shaft to be held for free rotation which forms a space of a dynamic pressure air bearing in collaboration with a fixed component. Amongst the ceramic materials, alumina having a flexural strength of 300 MPa and a thermal expansion coefficient of 0.7xc3x9710xe2x88x925/xc2x0 C. is the most popular material used for the above purpose due to its relative low cost. On the other hand, an aluminum alloy is generally used as one material of a polygon mirror which is fixed to the outer circumference of the rotary shaft, and the inner circumferential surface of the aluminum-alloy-made polygon mirror is fixed to the outer circumferential surface of the ceramic-made rotary shaft through a hardening insertion process or with an adhesion agent.
To fix the polygon mirror to the rotary shaft, the most common method is the hardening insertion method since it subjects the engagement to an increasing temperature associated with a high rotational speed greater than 40,000 rpm and heat generated by a polygon motor for rotating the polygon mirror. If the fixing is made with an adhesive agent, a fixing force of the adhesive agent will be lowered by a thermal stress due to a difference in a thermal expansion between the two components in the course of the rising temperature. For example, when an adhesive agent is used, the high speed rotary body will cause an imbalanced rotation at a temperature on the order of 90xc2x0 C. or higher and, as a result, vibration increases.
The hardening insertion process has a drawback, however, in that the ceramic-made rotary shaft produces a stress to constrict the shaft in a radial direction and, since the ceramic is vulnerable, the stress may cause a crack in the ceramic. The crack is a serious problem that may lead to breakage of the rotary body. Even a small crack residing within a surface layer of the inner circumferential surface that does not lead to any breakage may nonetheless degrade the quality of the deflecting functions of the polygon mirror, such as jitter and tracking accuracy properties.
To decrease a constriction stress produced under a relatively low temperature, one way is to reduce a radial margin of two components to be engaged through the hardening insertion. However, when the radial margin is reduced, the binding force weakens and, as a consequence, the engagement may be loosened and disengaged due to a relatively high temperature and a centrifugal force by a relatively high rotational speed. Therefore, the radial margin needs to be greater than a predetermined value.
On the other hand, one way to cope with a radial constriction force produced relative to the ceramic-made rotary body under a relatively low temperature environment is to increase a mechanical strength by using a silicon carbon having flexural strength of 500 MPa and a thermal expansion coefficient of 0.4xc3x9710xe2x88x925/xc2x0 C. or a silicon nitride having a flexural strength of 800 MPa and a thermal expansion coefficient of 0.3xc3x9710xe2x88x925/xc2x0 C. However, since the thermal expansion coefficients of these materials are smaller than that of alumina, the engagement of the two components can be loosened under a relatively high temperature causing an imbalanced rotation.
One example of the polygon mirror is described in Japanese laid-open patent publication No. 5-241090. This example appears to be successful in avoiding the generation of cracks in the ceramic by not using the hardening insertion. However, in this example, both the ceramic ring and yoke are integrally casted with an aluminum-made mirror surface member and an air void inherent in the casting is produced within the material. As a consequence, it becomes difficult to make the mirror surface of the polygon mirror at a high definition. In addition, the air void locally weakens the mechanical strength. Therefore, the above example is not suitable for the high speed polygon scanner which runs over 40,000 rpm.
In addition to the above-described problems of the engagement associated with the polygon mirror and the rotary shaft, the dynamic pressure air bearing motor has another problem of an engagement associated with a rotor magnet and a flange included therein. When the rotor is fixed to the flange with an adhesive agent, this causes a slight displacement between the components and leads to a problem referred to as a xe2x80x9cbalance change.xe2x80x9d Since it is extremely difficult in general to give a cross section of a perfect circle to a rotary component by machining, a perfect cross contact with an adhesive agent cannot be made relative to the entire circumferential surface of the rotary component. Therefore, when this rotary component is inserted into a hollow of a counter component, it causes an axial uneven engagement of the two components. In the engagement of the two components, portions of contact and non-contact are subjected to a thermal stress produced by a difference in a thermal expansion coefficient between the two components. Since this thermal stress is far greater than an adhesion strength by the adhesive agent, the fixing of the rotary component to the counter component cannot be maintained and consequently a slight displacement occurs at the portion engaged. In particular, when the polygon motor is driven for rotation over 30,000 rpm, the polygon mirror is exposed to a high temperature on the order of 80xc2x0 C. or higher and this problem becomes serious.
Japanese Laid-Open patent publication No. 2000-2851, for example, describes a polygon mirror scanner apparatus, that attempts to remedy the above-described problem of the engagement between the rotor magnet and the flange. In this apparatus, a rotor used as a rotary element is formed in a regular prism shape and each surface of the regular prism shape is used as a mirror. Thus, the rotor is used as a polygon mirror. The rotor is provided with a plurality of projections at the bottom thereof for engaging a multipolar magnet and determining the position of the multipolar magnet. This engagement is performed through a press-in insertion without using an adhesive agent. The multipolar magnet is fixed with the press-in insertion without using any adhesive agent. The multipolar magnet is disposed at a position to face a coil used as a fixed element with a predetermined distance. In this way, a brushless type direct current motor is structured.
To perform a press-in insertion, however, tight control of the insertion margin is generally required. In addition, the multipolar magnet is needed to have sufficient strength over the press-in insertion. For example, when an outer diameter is to be held, a material having a relatively high radial crushing strength is needed. Therefore, a metal magnet may be a potential candidate but, its heavy weight is a drawback which produces a relatively large inertial when rotating. Therefore, a metal magnet is not preferable for high speed rotation. In addition, a metal magnet is relatively expensive and has a low output. Furthermore, a sintered magnet is also not suitable for a relatively high speed rotation since it has a relatively low radial crushing strength and, when used, it can chip or fracture.
If a rotor magnet of any suitable material is successfully inserted into a counter component with the press-in insertion, the pressed-in portion of the rotor magnet may be loosened due to an increasing temperature when it is operated at a high speed rotation over 30,000 rpm. This may cause a balance change problem. Therefore, to insert the rotor magnet with the press-in insertion, one must optimize materials of both the rotor magnet and the counter component as well as a press-in insertion method.
This patent specification describes a novel high speed rotating apparatus. In one non-limiting embodiment, a novel high speed rotating apparatus includes a shaft, a rotary body, and a motor. The rotary body is configured to have a hollow into which the shaft is inserted and forms a dynamic pressure air bearing in a ring-shaped air space formed between the rotary body and the shaft. The rotary body includes first and second rotary members. The first rotary member is configured to have an inner circumferential surface facing the dynamic pressure air bearing which is held for rotation along an outer circumferential surface of the shaft by the dynamic pressure air bearing. The second rotary member is configured to have an inner circumferential surface facing an outer circumferential surface of the first rotary member and which is fixed to the outer circumferential surface of the first rotary member through a hardening insertion process. The motor is configured to drive the rotary body to rotate. With this structure, the hardening insertion process is performed so that a tensile strength of the first rotary member is greater than a tensile stress to be produced in an axial direction in the inner circumferential surface of the first rotary member.
In the above embodiment, a hardening insertion length of the second rotary member relative to the outer circumferential surface of the first rotary member may be 60% or greater of a total length of the first rotary member.
The first rotary member may be provided with a stress damper configured to reduce the tensile stress at a position where the tensile stress becomes maximum.
The second rotary member may include inner and outer rotors. The inner rotor is configured to have an inner circumferential surface which is fixed to the outer circumferential surface of the first rotary member through the hardening insertion process. The outer rotor is configured to have an inner circumferential surface which is fixed to the outer circumferential surface of the inner rotor through the hardening insertion process. With this structure, a thermal expansion coefficient of the first rotary member is smaller than a thermal expansion coefficient of the inner rotor, the thermal expansion coefficient of the inner rotor is smaller than a thermal expansion coefficient of the outer rotor, a Young""s modulus of the first rotary member is greater than a Young""s modulus of the inner rotor, and the Young""s modulus of the inner rotor is greater than a Young""s modulus of the outer rotor.
In the above embodiment, a hardening insertion diameter D [mm] and a hardening insertion length xcex4 [mm] associated with the first and second rotary members satisfy a relationship;
(Dxc3x970.0014) less than xcex4 less than (Dxc3x970.030), under a Temperature of 20xc2x0 C.
The first rotary member may have an outer diameter 1.2 times greater than its own inner diameter.
The shaft and the first rotary member may be made of ceramic, and the second rotary member and the inner and outer rotors may be made of metal.
This disclosure further describes a novel high speed rotating apparatus. In one non-limiting embodiment, this novel high speed rotating apparatus includes a fixed shaft, a stator core, a rotary body, and a rotor magnet. The stator core is configured to be fixed inside the fixed shaft. The rotary body is configured to have a hollow into which the fixed shaft is inserted, the rotary body including a fixing portion. The rotor magnet is configured to be fixed to the fixing portion of the rotary body to face the stator core. In this high speed rotating apparatus, an outer circumferential surface of the rotor magnet is cut away by an edge of the fixing portion when the rotor magnet is inserted into the fixing portion of the rotary body through a press-in insertion.
The rotary body may be configured to form a dynamic pressure air bearing in a ring-shaped air space formed between the rotary body and the shaft.
The rotor magnet may include a bond magnet using a plastic binder and the rotary body is made of aluminum alloy.
The bond magnet of the rotor magnet may have a thermal expansion coefficient substantially equal to a thermal expansion coefficient of the aluminum alloy.
The above-mentioned disclosure further describes a novel method of making a high speed rotating apparatus. In one example, this novel method includes the steps of fixing and inserting. The fixing step fixes a rotary sleeve to a flange through a hardening insertion process. The inserting step inserts a rotor magnet into a skirting portion of the flange through a press-in insertion such that an outer circumferential surface of the rotor magnet is cut away by an edge of the skirting portion of the flange.
The above-mentioned method may further include a step of magnetizing the rotor magnet after the inserting step.
This disclosure further describes a novel polygon scanner apparatus. In one example, this novel polygon scanner apparatus includes the above-described high speed rotating apparatus, in which the high speed rotating apparatus further includes a polygon mirror configured to have a regular polygonal mirror.
This disclosure further describes a novel polygon scanner apparatus. In one example, this novel polygon scanner apparatus includes the above-described high speed rotating apparatus, in which the high speed rotating apparatus further includes a polygon mirror configured to have a regular polygonal mirror.